Methods for assembling prepreg stacks having exact weight for producing smc components

ABSTRACT

A method for manufacturing SMC components from an appropriately adapted amount of a fibrous reactive synthetic resin which is provided in the form of defined blanks of prepregs. The prepregs are placed in a defined position into a heated mold of a molding press, the heated resin/fiber mass is flow-molded in the closing mold to form the SMC component, said component is thermally cured and subsequently removed from the mold. Two different approaches are taken, one being intended for introducing the resin mass into the mold as multiple layers and the other for introducing the resin mass as a single layer.

[0001] This application claims the prirority of German Patent DocumentNo. 101 45 308.6, filed 14 Sep. 2001 and PCT/EP02/09095 filed 14 Aug.2002 the disclosure of which is expressly incorporated by referenceherein, respectively.

FIELD OF THE INVENTION

[0002] The invention relates to a method for producing SMC componentsfrom fibrous, reactive prepregs.

BACKGROUND OF THE INVENTION

[0003] An article by R. Brussel and U. Weber “SMC-Teile vollautomatischherstellen” [Fully automatic production of SMC components], published inthe journal Kunststoffe, year 79 (1989), pages 1149-1154—cited hereafteras [1] for short, describes a method for forming SMC components.

[0004] According to the literature reference [1], the production of SMCcomponents starts with a specific amount of a mixture of reactivethermosetting synthetic resin and fibers that is adapted in its weightto be appropriate for the finished component. To be precise, the adaptedamount of raw material is obtained by cutting out blanks of a specificsize and shape from a prepreg web supplied in roll form and by layingthe blanks together to form a prepreg stack. Such a prepreg stack isplaced exactly in position in an opened mold of a press. The mold isheated to a temperature at which the reactive synthetic resin chemicallyreacts and sets. By initially slow closing of the mold located in thepress, the raw material introduced is at first merely heated, wherebythe synthetic resin becomes soft and free-flowing. Subsequently, themold is closed with a controlled force and speed, the softened rawmaterial flowing away to the sides and thereby completely filling thecavity of the mold. After this filling of the impression, the mold iskept closed for a time with a defined force, so that the synthetic resincan fully react and cure. Only then can the mold be opened and thefinished SMC component removed from it.

[0005] In the article [1] cited at the beginning, reference is madeinter alia to a varying basis weight of the prepregs. In spite of allthe efforts of the prepreg manufacturers, according to [1] even today itis still not possible for the prepreg webs to be manufactured withsufficient accuracy in the basis weight. Therefore, in preparation foreach manufacturing step of an SMC component, it must be ensured that themass of prepregs introduced into the mold is always the same, at leastwithin a certain tolerance range. The higher the quality requirementsimposed on the finished product, the less the resin mass introduced mayvary about a desired value. In [1] it is mentioned that the problem ofthe varying basis weight of the prepreg web, and the consequent problemof exact feeding of the raw mass, could be overcome if the qualityrequirements imposed on the finished SMC molding could be reduced. If,however, the SMC components to be manufactured are thin-walled shellcomponents with high quality requirements, the mass of the raw materialto be introduced should wherever possible be fed in with a low range ofupward and downward variation in comparison with a setpoint selection.If the amount of raw material introduced is too small, this causes theformation of surface roughnesses and also thin and weak points in thecomponent, which in an extreme case could become perforated. If, on theother hand, too much raw material is fed into the mold, the wallthickness becomes too great, at least locally, which under somecircumstances leads to warping of the component; in any event,components with excessive material are not dimensionally stable enough.Furthermore, in the case of overfeeding, material swells out along theparting line of the mold, which leads not only to excessive flash andcorresponding extra work to remove the flash, but also to increasedsoiling of the mold and consequently an increase in the secondary workof “mold cleaning”; that is overfeeding leads overall to a reduction inproductivity.

[0006] In the case of the automated method for manufacturing SMCcomponents described in the literature reference [1], the blanksarranged in layers to form a prepreg stack as a raw mass are allrectangularly shaped and all have the same width in one direction, lyingtransversely to the prepreg web, that is the width of the prepreg webitself trimmed at the edges. The blanks are produced by cutting acrossthe prepreg web, using a pneumatically driven high-speed cutter that ismoved transversely over the prepreg web, which is supported at thelocation of the cut by a narrow profile. The high-speed cutterpresumably leaves the prepreg web to be cut on the underside and entersa longitudinal slot in the supporting profile. To compensate for achanged basis weight of the prepreg web, the rectangular dimension ofthe blanks in the longitudinal direction of the prepreg web is used. Formonitoring the target weight to be maintained of the prepreg stack, itis not the cut-off blanks that are weighed but the finished SMCcomponent. Depending on the deviation of the finished weight of the SMCcomponent from a desired weight, the blanks are cut longer, shorter orthe same for the next SMC component to be produced. A fundamentaldisadvantage of the control strategy known from [1] for maintaining thedesired weight of the raw mass to be introduced is that a controlintervention for correcting the actual controlled variable—raw mass forthe component n—is made dependent on the desired/actual deviation of avariable other than the measured variable—that is the finished componentmass of the component n+1. The measured variable “finished componentmass of the component n+1” does not by any means have to berepresentative of the actual controlled variable “raw mass for thecomponent n”. The method described in [1] attempts to record or predictpossible differences between the controlled variable and the measuredvariable by continuously recording the thickness of the prepreg web.Because of the differences between the measured variable on the one handand the controlled variable on the other hand, a high proportion of theSMC components manufactured according to deviate from the desired weightaimed for; the control strategy known from [1] only works on the basisof such a deviation. Apart from this, in the method according to [1] theblanks have to be rectangular, with a width corresponding to the widthof the prepreg web. However, this prerequisite can only be allowedoptimally in terms of the method for a restricted spectrum ofcomponents.

[0007] In conventional methods for manufacturing series of SMCcomponents, often performed manually, the mass of prepregs introducedinto the mold is individually weighed, which likewise takes placemanually and constitutes a great obstacle to automation of the process.This usually involves cutting out rectangular blanks with a sharp knifefrom a virtually endless prepreg web on a steel base and weighing them.If the desired weight of a blank to be introduced into a prepreg stackis too great, an edge strip is cut off on one longitudinal side of ablank or a triangular piece is cut off at a corner, whereby the desiredweight is achieved approximately but not exactly. In particular,however, the desired shape of the blank is greatly changed by such acorrection, which has disadvantageous effects on the molding process andthe component quality. If, on the other hand, the desired weight of ablank is too low, the next-following blank is cut somewhat larger thanthe desired shape or a small trimmed-off part of an earlier correctiveoperation is added. These types of correction have disadvantageouseffects on the subsequent molding operation and the component quality.Moreover, this manual weighing of the amount of raw material means thatthe desired weight is only approximated with a very great range ofvariation, which is scarcely any less than the weight variation of theprepregs themselves. For this reason, in the production of SMCcomponents with manual weighing of the raw material there are arelatively high number of reject components and relatively considerablequality variations.

[0008] EP 461 365 B1—cited hereafter as [2] for short—discloses a methodfor manufacturing plastic moldings from thermoplastic material in whichan amount of heated and softened thermoplastic material appropriatelyadapted in weight is placed into an opened mold of a press, the moldingcompound is forced to flow into the cavity of the mold by closing themold and subsequently the workpiece still located in the mold is cooledand finally removed from it. The special feature of the method describedin [2] is the preparation of the heated thermoplastic material in a flatpreform already appropriately adapted approximately to the shape of thecavity of the mold, with the distribution of the compression moldingcompound within the preform also already having been approximatelyadapted to the requirements of the cavity of the mold. For this purpose,a thin, wide strand of extrudate of hot molding compound is deposited onthe heated and reversibly drivable conveyor belt of a belt weigher andat the same time weighed. The strand of extrudate is deposited on theconveyor belt in a meandering manner and with a variable massdistribution on account of a slow oscillating motion of the conveyorbelt in the conveying direction or counter to it and on account of aspecific belt speed, which may deviate from the extrusion speed. Also inthe case of this method for the compression molding of thermoplasticmaterial, the molding compound to be introduced into the cavity of themold is to correspond exactly to a desired weight, in order on the onehand to ensure complete filling of the cavity and on the other hand topermit complete closing of the mold without excessive formation offlash. In the case of the method shown in [2], this is achieved by thestrand of extrudate deposited on the belt weigher being continuouslyweighed. When, toward the end of the formation of a preform, part of thestrand of extrudate is still hanging from the extruder die and not theentire molding compound intended for the preform is exerting its weighton the weigher, the extruded strand is cut off just before the desiredweight is reached, i.e. when a certain weight threshold is reached. Ifthe weight of the preform that is then completely on the belt weigherlies within a predetermined tolerance range, it is passed on to adownstream conveying devices, which deposit the preform into the openedmold. If, on the other hand, the formed preform is too heavy or toolight, it is rejected and its molding compound is recycled. The weightthreshold for triggering the severing of the strand for thenext-following preform is also correspondingly corrected, i.e. in thecase of an excessively heavy preform the weight threshold is changed inthe direction of a lower threshold weight, and vice versa. However, thistype of control of the weight of the molding compound to be introducedinto a mold cannot be transferred to the processing of fiber-reinforcedthermosetting materials, i.e. it cannot be transferred to the portioningof prepreg blanks.

[0009] Japanese laid-open patent application JP 10 044 153 A, citedhereafter as [3] for short, discloses a method for preparing prepregblanks for the manufacture of SMC components. This involves processingindividual pieces of material web of a length adequate for forming anumber of rectangular blanks and of a width which coincides with thewidth of the blanks. At the beginning of the processing of a materialweb, a first rectangular blank of a known length is cut off, this firstblank is weighed and this is used to determine the weight per unitlength of the material web. On the basis of this weight per unit length,assumed to be sufficiently constant within the piece of material web tobe processed at the time, a length of blank is mathematically fixed forthe blanks subsequently to be cut off with the same surface area fromthe material web, with which length the blanks have a weight whichcoincides approximately from one to the another and also correspondswith sufficient accuracy to the weight required for the workpiece to beproduced. Subsequently, the other pieces of material web aretransversely cut in a way corresponding to this fixed specification intopieces of the same length and the blanks prepared in this way areprocessed one after the other in an SMC molding press for workpieces. Adisadvantage of this procedure is that it is only possible in this wayto process relatively short pieces of material web, for which the basisweight or the weight per unit length can be regarded as constant withsufficient accuracy over the entire length of the piece of web.

SUMMARY OF THE INVENTION

[0010] An object of the invention is to provide a method of producingSMC components wherein, in spite of a varying basis weight of theprepreg web, the desired weight prescribed for the prepregs can bemaintained with high accuracy for every production cycle of an SMCcomponent, without significantly changing the shape of the individualblanks.

[0011] This and other objects and advantages are achieved according tothe invention in the following embodiments.

[0012] In an embodiment, in every working cycle a reference blank withconstant shape and size is cut to size and separately weighed each time.The weight and size of the reference blank and the aimed-for totalweight of the prepreg stack are used to determine mathematically thesurface-area size of corrective parts which are cut to obtain thedesired weight of the resin mass to be introduced. This is based on theassumption that the basis weight of the prepreg web changes only by anegligible amount in the direct vicinity of the location at which thereference blank has been cut out from the prepreg web. Such anembodiment, which is based on a resin mass comprising a stack ofmultiple layers, the further prepreg layers are regarded as correctiveparts and the size to be maintained by all of them is determined. Theseother blanks are then cut out from a region of the respective prepregstack lying directly adjacent to the reference blank in the prepreg weband formed into a stack.

[0013] In another embodiment wherein the prepreg is to be introduced asa single layer, a reference part is cut out with excess size, this isweighed and the excess in terms of weight is then cut off in acorresponding surface area.

[0014] Other objects, advantages and novel features of the presentinvention will become apparent from the following detailed descriptionof the invention when considered in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 shows a schematic overall view of an installation for themethod in a plan view,

[0016]FIG. 2 shows the cutting table with the outline on a prepreg webfor the cutting to size of the parts of a seven-part prepreg stack of afirst exemplary embodiment,

[0017]FIG. 3 shows an auxiliary device set up on a weigher for weighingthe reference blank and for preparing a prepreg stack obtained accordingto FIG. 2,

[0018]FIG. 4 shows the cutting tool arranged on the hand joint of anindustrial robot, with a circular saw blade, which performhigh-frequency rotary oscillating movements, for cutting up the prepregweb supported by a glass plate,

[0019]FIG. 5 shows an enlarged detail of the cutting intervention of thecircular saw blade into the supported prepreg web and

[0020]FIG. 6 shows the cutting to size of a single-layer useful blank ofa predetermined weight from a reference blank of varying basis weight asa second exemplary embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMODIMENT

[0021] The method on which the invention is based for manufacturingseries of SMC components is to be briefly explained on the basis of thediagram of the method according to FIGS. 1 and 2. The SMC components aremanufactured from a fibrous, reactive resin mass, which is provided inthe form of a virtually endless prepreg web 22 wound up into a supplyroll 1 as an intermediate. The fibers contained in the prepreg web aregenerally glass fibers; in the case of heavy-duty SMC components, carbonfibers or Kevlar fibers may also be integrated. The fibers are cut andhave a length of approximately 1 to 5 cm. To maintain the reactivity ofthe synthetic resin in the prepreg web 22, the latter is covered on bothsides with a protective film 26, which is pulled off and rolled up toform a separate roll 2 only shortly before the processing of theprepreg. As can be seen more clearly in FIG. 2, the protective film isdeflected counter to the processing direction of the prepreg to the roll2 via a reversing rod 12, located in the vicinity of the cutting table3. The side edges of the prepreg web are unsuitable for furtherprocessing and must be cut off. The lateral waste strips 28 are likewisedeflected via reversing rods 13 into waste containers 14.

[0022] Alternatively, when the cutting tool described further below isused, it is also possible for the prepreg webs provided with anadhesively attached protective film to be cut. For example, the edgestrips 28 (FIG. 2) can be cut off before the protective film 26 ispulled off. In some embodiments, it may be desired to cut the blanks tosize with the protective film, to allow them to be stacked up beforefurther processing and only processed further at a later point in time.The protective film adhering to each of the blanks readily prevents theblanks that are stacked or deposited in an imbricated formation fromsticking together. Conventionally separate sheets of film were requiredas an intermediate layer, causing extra costs.

[0023] The following description is based on an embodiment, in which theprotective film is pulled off and wound up as one before the cutting ofthe prepreg web. This embodiment has the advantage that it can bechecked more easily whether the protective film has detached itselfcompletely from the prepreg web. Remains of film adhering to the blanksare detrimental to the further processing process and to the strength ofthe molding to be manufactured.

[0024] In an embodiment of the prepreg web 22 is cut up on the cuttingtable 3 provided with a very hard support. Blanks of a defined shape andsize are cut out from the prepreg and stacked up to form a multi-layerprepreg stack web of a specific number of layers and arrangement oflayers. The trimmed-off parts produced thereby, which cannot be used anyfurther, are removed into a corresponding waste container 4. The cuttingto size may in principle be performed manually with a sharp knife and asteel rule. In the case of the exemplary embodiment represented in thefigures, however, a mechanized and automated cutting to size by means ofa cutting robot 5 is provided. This embodiment is discussed in moredetail below.

[0025] On a separate weighing and stack-forming device 6—see also FIG.3—the blanks cut by the robot 5 on the table 3 are stacked up to form aprepreg stack 31, the blanks being handled and moved by a handling robot7, which for its part is equipped with a prepreg gripper 27 designedspecifically for this task and this substrate. Once the prepreg stack 31has been formed in an appropriate shape for a new workpiece, it isplaced by the handling robot in a defined position into a heated mold 35of the molding press 8.

[0026] The mold is closed by the press until the molding surface of thecavity is in contact with the placed-in prepreg stack and is clamped inthe closing sense by a defined, initially small force. The contact withthe hot mold causes the resin mass to be heated and softened as aresult. On account of the closing force of the mold 35, the resin massbegins to flow and, as a result, fills the cavity of the increasinglyclosing mold. The mold is subsequently held in the closed state with anincreased force for a certain time, the resin mass thermally curing.Once this curing time has elapsed, the press 8 opens the mold, with thefinished SMC component remaining in the lower, fixed mold half. The SMCcomponent can be removed from the press and deposited in a coolingstation 11 by a removal robot 9, which is provided with a removal tool39. While the cutting and handling robots 5 and 7 prepare a new prepregstack, the opened mold is cleaned by two cleaning robots 10, so that itis ready for receiving a new prepreg stack.

[0027] The described method for producing the SMC components requiresthat the resin mass introduced into the mold coincides with a specificdesired weight. The cavity of the mold should be completely filled bythe resin mass introduced, but on the other hand there should not be toomuch resin mass in the cavity, because otherwise the mold cannot closecompletely and the formed SMC component is not molded to the truedimensions. Furthermore, an unnecessarily large amount of resin massswells out at the parting line of the mold, which makes the cleaningoperation more difficult. Unfortunately, the prepreg web 22 hasinadmissibly high variations in the basis weight, which cannot beavoided in the manufacture of this intermediate. Consequently, toachieve the prescribed desired weight, it is not possible to use blanksof a constant surface area from one working cycle to the next. Rather,special efforts have to be undertaken to achieve the target weight ofthe resin mass to be introduced in each working cycle.

[0028] To be able to maintain with great accuracy the desired weightprescribed for the prepreg stack to be introduced for each productioncycle of an SMC component, without significantly changing the shape ofthe individual blanks, the present invention provides a method forcutting the blanks of the prepregs to size. This is based on the largelycorrect assumption that the basis weight of the prepreg web changes onlyby a negligible amount within a surface-area region required for theamount of resin mass needed for an SMC component.

[0029] On the basis of this finding, the blanks 24, 25 to be used forthe prepreg stack 31 of a specific SMC component are not only cut outfrom one and the same prepreg web 22, but also cut out from it directlyadjacent to one another. Furthermore, a special blank, a reference blank24, with a shape that is always constant and the same surface-areacontent Fr for all the prepreg stacks following one another is cut tosize and separately weighed each time after the cutting to size, and itsactual weight G_(r,actual) is determined. As a result, the local, actualbasis weight of the prepreg web is to a certain extent known. Apart fromcomprising the reference blank 24 of a constant surface area, therequired prepreg stack comprises further blanks 25 formed with variablesurface areas. These can be specifically dimensioned in their surfacearea in such a way that the total weight of the prepreg stack can betrimmed exactly and in a single operation to the desired weight that isto be maintained.

[0030] The respective weight G_(r,actual) and the surface-area contentF_(r) of the reference blank 24 as well as the predetermined totalweight G_(g) of all the blanks 24, 25, that is always the same for allthe prepreg stacks 31 following one another, are used to determine thesurface-area content F_(u) to be maintained by all the other blanks 25which individually correspond to the weighed reference blank 24, inaccordance with the relationship

F _(u) =F _(r)·(G _(g) /G _(r,actual)−1)

[0031] or in accordance with a relationship which is derived therefromand identical in principle. With the knowledge of the magnitude of thesurface-area content F_(u) of the other required blanks 25, the lattercan be cut out from the prepreg web in an exactly specific manner withrespect to their weight together. These blanks are cut out from a pieceof the surface area lying directly adjacent to the reference blank 24 inthe prepreg web 22, appropriately adapted in shape and size and with thesurface-area content F_(u). The prepreg stack 31 assembled with theweighed reference blank 24, of a constant surface area, and with theother blanks 25, dimensioned individually in surface-area content, has atotal weight G_(g) that coincides with the desired weight to within afew tenths of a percent. In a laboratory trial carried out by theapplicant for the invention, it was possible to maintain thepredetermined desired weight of the prepreg stack to within a range ofvariation of ±0.3%. The prepreg stack of constant weight formed in thisway can consequently be readily placed in a defined position into themold 35 ready to receive it of the molding press 8 for furtherprocessing.

[0032] The continuous weighings of reference blanks of the same surfacearea, carried out when the method according to the invention isperformed in a series mode, incidentally also provide a reliable volumeof data with respect to the variation in the basis weight of the prepregweb in the longitudinal direction of the web. The very dense set of datagenerated can be evaluated in various respects. For example, the basisweight of the prepreg web in the longitudinal direction of the web canbe printed out as a longitudinal profile, i.e. as a line trace; the meanbasis weight, the standard deviation and the maximum deviation from themean value can be determined. These data allow reliable quality controlor quality monitoring of the prepreg webs delivered.

[0033] As already mentioned, for each workpiece the total weight G_(g)of the prepregs is entered as a desired value into a mathematicaloperation which is then used for cutting the prepreg parts to sizeexactly and individually for each workpiece. The invention readilyallows this desired value to be slightly changed if appropriate or to beadapted to new findings or circumstances. The desired weight can becorrected in the course of series production from a value X to a valueof, for example, X+0.5% or, for example, to a value X−1.3%. With theinput of the new desired weight, the actual total weights of the prepregstacks manufactured after the change are then also correspondinglyhigher or lower, to be precise likewise with the accuracy mentioned of±0.3%. The invention therefore allows sensitive and exact selection ofthe total weight of the prepregs that are to be placed into the mold 35.

[0034] Once the prepreg stacks have been assembled on a weigher, it isreadily possible also to determine exactly the actual total weight ofthe finished prepreg stack by weighing, and to fix it for eachworkpiece. This not only allows monitoring of the method according tothe invention of weighing the prepreg webs. The actual weights of thefinished prepreg stacks that are fixed individually for each workpiecealso provide important data for quality monitoring of the production ofmoldings.

[0035] Based on the admissible assumption made that the gradient of thebasis weight within the prepreg web 22 is small, it follows that thebasis weight is constant with sufficient accuracy within the amount ofsurface area needed for a prepreg stack. This in turn leads to therecommendation to cut out the reference blank 24 and the other blanks 25of each prepreg stack next to one another from the prepreg web,transversely to the longitudinal direction of the prepreg web, in such away that the space required in the longitudinal direction of the prepregweb is as small as possible.

[0036] In the case of a prepreg stack in which the other blanks 25 areformed such that they are rectangular and also congruent, as providedfor example in the case of the exemplary embodiment represented in FIGS.2 and 3, one dimension l of the side of the rectangle of the otherblanks 25 is left unchanged for all the prepreg stacks 31 following oneanother. Only the dimension b transverse thereto of the side of therectangle of the other blanks 25 is dimensioned individually for theprepreg stack in question according to the technical teaching of thepresent invention. In the case of the exemplary embodiment representedin FIG. 2, with six other blanks 25, the unchanged longitudinaldimension l of the side of the rectangle of the other blanks is alignedparallel to the longitudinal direction of the prepreg web 22. Theindividually dimensioned width dimension b of the side of the rectangleof the other blanks is aligned transversely to the longitudinaldirection of the prepreg web 22. In the simplified case described here,the width b of the other blanks 25 can be individually fixed inaccordance with the relationship b=F_(r)·(G_(g)/G_(r,actual)−1)/l·n,where n denotes the number of other blanks 25. Once the actual weight ofthe reference blank 24 has been determined on the weigher 15, this valueis automatically entered in digitized form into the control system orthe movement program of the cutting robot 5, which then cuts out theother blanks 25 with the individual width b from the prepreg web 22 onthe basis of this input.

[0037] The space requirement evident from FIG. 2 for the seven blanks 24and 25 shown there cannot be evenly distributed over the width of theprepreg web 22. In the region of the reference blank 24, only materialof the dimension A is used up in the longitudinal direction of the web,whereas on the opposite side of the web material of the significantlygreater dimension 2·l is used up. To compensate for this, it isexpedient to change over the sides on which the reference blank 24 andthe other blanks 25 are arranged for the next-following cutting-to-sizeoperation, so that the prepreg is used up evenly on the right and left.However, this has to be correspondingly taken into consideration in theprogramming of the sequence of movements of the cutting robot 5.

[0038] The usable width of the prepreg web 22 is at least slightlygreater than the width B of the reference blank 24 plus three times thegreatest width b of the other blanks 25. As a result, an edge strip 30is generally obtained at one edge of the prepreg web as cutting loss andis discharged into the waste container 4. Only in the extreme case of anextremely low basis weight of the prepreg web may this edge strip lossbe negligible.

[0039] For the sake of completeness, another possibility should also bepointed out, that of trimming the weight of the prepreg stack to thedesired weight value in a single operation by suitable surface-areadimensioning of the other blanks. The possibilities mentioned belowlargely depend on the type and shape of the SMC component to bemanufactured and the related question as to the extent to which the formof the prepreg stack may vary from workpiece to workpiece. To beprecise, it is conceivable to cut out from the prepreg web a certainnumber, for example four, of the other blanks 25 for each prepreg stacklikewise with the same surface area, i.e. with a constant length andwith an always constant width, and merely to dimension the remainingnumber, in the example two, of the other blanks 25 individually inwidth. These two individually dimensioned blanks 25 would of course varymuch more in their width dimension b than if the weight compensationwere evenly distributed over six blanks 25. In the extreme case, itwould even be possible to use only one of the other blanks for such aweight compensation, which in the case of a two-layer prepreg stackwould in any case be unavoidable.

[0040] In the case of the exemplary embodiment represented in FIGS. 2and 3, the prepreg stack to be formed altogether comprises seven blanks,that is a particularly large reference blank 24 and six much smallerother blanks 25, which are stacked up in two small stacks lying next toeach other on the reference blank 24 lying at the bottom. Irrespectiveof the arrangement of the reference blank in the prepreg stack to beformed, however, a blank that is as large as possible should be selectedto form the reference, so that the weight determined is also reliablyrepresentative of the basis weight of the prepreg web in the region ofthe web end that is being worked on at the time. The reference blankshould expediently have a size of approximately 20 to 60% of the totalsurface area of all the blanks of the prepreg stack 31. If it is toosmall, the weight and the surface area do not represent the local basisweight with sufficient accuracy. If, on the other hand, the referenceblank is too large, it may be that the weight compensation is notsuccessful in every case with the relatively small other blank or blankswithout trimming them excessively in the extreme case of a very highbasis weight. In such a case, it is then better to take away at leastpart of the excess prepreg weight from the reference blank itself. Thisis to be discussed once again further below in connection with a furtherexemplary embodiment according to FIG. 6.

[0041] In the procedure of the method it is favorable if the blank lyingat the bottom in the prepreg stack 31 is adequate in size to allow it tobe selected as the reference blank 24. After weighing the referenceblank, no further handling operations are then necessary with it, i.e.the other blanks 25 can be stacked on the reference blank still lying onthe weighing plate 16 of the weigher 15 to form the prepreg stack 31.With blanks that are substantially congruent, the lowermost blank istherefore chosen as the reference blank and is cut to always the samesize of surface area. In the case of stacks with, for example, fiveidentical prepregs, this would be approximately 20% of the total weight.In the case of six or more prepregs in a stack, the two lowermostprepregs, for example, can both be chosen to be the reference blank, cutto always the same size of surface area and weighed together.

[0042] In the case of the exemplary embodiment represented in FIG. 3,the reference blank 24 is not only weighed when it is placed onto theweigher 15, but at the same time also pre-formed in a specific way, asexpedient later for placing the finished prepreg stack 31 into the mold.For this purpose, fastened on the weighing plate 16 of the weigher is astacking device 17, which permits staged pre-forming of the referenceblank by the handling robot and the prepreg gripper. The other blanks 25are stacked up on the lower or upper portion of the reference blankdeposited in stages.

[0043] In principle, the invention can also be put into practice in amanually operated procedure for the method, in which for example thecutting out of the blanks is also performed by means of a hand-heldknife and steel rule on a steel base and in which the blanks aremanually handled by the worker. This manner of working is alsooccasionally still encountered today in the series production of SMCcomponents. The reference blank 24 could be cut relatively precisely byusing a template. Templates of different shapes could also be used forthe other blanks 25, it being automatically output after the weighing ofthe reference blank which template from a finely graduated set is to beused. Cutting to size by using punching tools, as are used for thecutting to size of leather in a flat press, is also conceivable, ifappropriate with the assistance of vibrators. For cutting the otherblanks to size with flexibility in their surface area, a set of finelygraduated punching tools would then have to be kept available. Dependingon the computed result, an individually specified punching tool from theset would have to be issued and placed onto the prepreg web for punchingout a blank.

[0044] However, in spite of all the care taken, manual cutting to sizeof the prepregs entails the risk of a greater surface-area tolerance andconsequently weight tolerance. Quite apart from this, this strenuouswork in the direct proximity of noxious fumes from the prepreg webs isonly admissible for any time with a protective mask and is thereforeeven more strenuous or laborious. To avoid such manually causedinaccuracies and the strenuous work, it is recommended to cut out boththe reference blank 24 and the mathematically determined cut-to-sizeareas of the blanks 25 by means of a robot-guided cutting tool.

[0045] Under some circumstances, a sharp cutter with an exchangeableblade could be used as such a robot-guided cutting tool—in a way similarto in the case of manual cutting to size—said cutter being moved throughthe prepreg with a drawing cut, i.e. at a shallow angle, although itwould be necessary to monitor that no fiber strands attach themselves tothe cutting edge and disturb a clean cut. Because of this problem, ahigh-frequency rotary oscillating circular saw blade 21, which performssmall rotational displacements around a stationary central position, isrecommended in the present case as the cutting tool for the automatedcutting to size of the blanks by means of cutting robot 5. During thecutting to size, the prepreg web 22 is supported by a smooth, continuousbase that is free from joints, in the form of a thick glass plate 23,which is harder than the cutting teeth of the circular saw blade.

[0046] At the hand joint 18 of the cutting robot 5, a drive motor 20 forthe circular saw blade 21 is secured by means of a holding angle in sucha position that the hand joint axis 19 crosses the axis of the rotaryoscillating movement of the circular saw blade. The drive motor sets thecircular saw blade in rotary oscillations with approximately 20,000rotational displacements per minute via an integrated displacement gearmechanism. The cutting tool is guided along the desired cutting line insuch a way that the circumference of the circular saw blade touches theglass plate 23 with a small force during the cutting. The quite smallrotary oscillating displacements h performed by the circular saw bladeare indeed greater than the tooth pitch t, but smaller or slightlygreater than the thickness s of the prepreg web 22. The rotaryoscillating circular saw blade acts in a way similar to a compass saw,but with two fundamental differences. On the one hand, the sawing toolhas a displacement along a circular path which tangentially enters thematerial being cut and does not leave the material being cut on theunderside; the cutting displacements in the form of circular arcs areoriented at a very shallow angle with respect to the plane of theprepreg. On the other hand, even in the loosely lying state, thematerial being cut cannot follow the high-frequency oscillating movementbecause of inertia, so that the prepreg resting loosely on the glassplate can be cut through without any trouble. The advantage of thiscutting tool is not only trouble-free and low-wear working when cuttingprepregs to size, but also the possibility of being able to carry outcuts along tight curves with precision.

[0047] Also to be briefly discussed below in connection with theexemplary embodiment represented in FIG. 6 is a variant of the methodfor the series production of SMC components in which an in principlesingle-layer useful blank 33 of a prepreg 22′ is used and it is placedin a defined position into the heated mold of a molding press. Thefurther sequence of the method for the manufacture of SMC components isthe same in principle here as already described further above. Even ifin the actual case a smaller blank or else a number of them should beplaced onto the useful blank 33 locally and in a defined position, thiscase is also intended to be implied in the description andrecommendation which follows, although this additional, smaller blank isnot specifically mentioned below.

[0048] Even when useful blanks that in principle comprise a single layerand are of constant weight Gn are used, at first a reference blank 32 iscut to size with a shape and surface content Fr that is always the samefor all the SMC components following one another and separately weighedeach time after cutting to size, the actual weight G_(r,actual) in eachrespective case being determined. The shape and surface-area content Frof this reference blank are chosen such that the latter protrudes beyondthe useful blank on all sides in every case. Even assuming an extremelylow basis weight of the prepreg web 22′, the reference blank is largeenough to allow the useful blank 33 to be cut out from it with theprescribed weight G_(n) of the useful blank 33. In any event, a more orless large waste piece 34 is obtained when cutting back the referenceblank, i.e. when cutting to size the useful blank 33 from it.

[0049] In the case of the exemplary embodiment represented in FIG. 6, arectangular shape with the side lengths A′ and B′ is chosen for thereference blank 32, the width dimension B′ corresponding to the usablewidth of the prepreg web 22′. The reference blank can then be cut tosize by a straight cut taken transversely to the longitudinal directionof the prepreg web at the distance A′ from the previous end edge. Itjust has to be ensured that the surface-area content F_(r) of all thereference blanks are the same as one another with an error deviation ofvery few tenths of a percent. The cutting table 3′ used for cutting thereference blank 32 is at the same time formed as a weigher, i.e. theglass plate 23 forming the table top is at the same time the weighingplate of a weigher. For weighing the cut-free reference blank, the endof the prepreg web 22′ must be temporarily lifted off the cutting table3′ by the handling robot 7 or by another, more simple auxiliary device.Alternatively, it is also possible to restrict the weighing plate onlyto a partial region of the cutting table and, for weighing only thereference blank, to lower the weigher to such an extent that the end ofthe prepreg web is no longer touching the weighing plate, because it isheld by the surrounding table top.

[0050] The respective weight G_(r,actual) and the surface-area contentF_(r) of the reference blank 32 and also the predetermined weight G_(n)of the useful blank 33 are used for determining the surface-area contentF_(a) of the excess in terms of surface area of the reference blank 32in comparison with the surface-area content F_(n) of the useful blank33, i.e. the size in terms of surface area of the waste piece 34. Forthis, the relationship F_(a)=F_(r)·(1−G_(n)/G_(r,actual)), or arelationship which is derived therefrom and identical in principle, isused.

[0051] Once there is knowledge of the surface-area content F_(a) of thewaste piece 34, this item of data can be automatically entered in asuitable, for example digitized, form for each workpiece into thecontrol system of the cutting robot 5. Stored in the robot controlsystem is a finely graduated set of movement lines for guiding a curvedcut, each individual movement line being assigned a specific surfacearea F_(a). Three of these cutting lines are indicated in FIG. 6. Inaccordance with the output of a specific F_(a) value, the associatedmovement program is activated in the control system of the cutting robotand the cutting robot is moved in accordance with it. The hatched regionlying outside the actual cutting line represents the waste piece 34 tobe removed. If the reference blank is very heavy, a waste piece 34 witha large surface-area content F_(a) is cut off, in the case of a lightreference blank the converse case applies. In any case, the waste piecesto be cut off are similar in their shape and in any event a piece ofsurface area coinciding in shape and size with the desired useful blank33 with the weight G_(n) remains and can be placed into the mold.

[0052] In an embodiment of the method according to FIG. 6 that, if ithas been cut to size by using an industrial robot, the reference blankis no longer moved after the cutting to size, i.e. the reference blankshould not, for weighing purposes, be taken off the base on which it wascut or be moved, because otherwise the reference to the system of thecutting robot is lost. This is also the reason why the cutting table isformed at the same time as a weigher or as a weighing plate.

[0053] It may happen under some circumstances that, because of a localthickening on a workpiece, a smaller blank also has to be placed locallyonto the useful blank. This additional blank would be ignored in themethod previously described in connection with FIG. 6 for the weightcorrection of the resin mass to be introduced by cutting back thereference part 32 specifically in terms of its surface area. If thisadditional part were always cut to size with the same surface area, acertain error would be included in the predetermination of the totalweight of an amount corresponding to its proportionate size. In orderhowever to allow such a smaller blank also to be included in thepredetermination of the total weight that is to be maintained,nevertheless only the weight of the bottom useful blank would have to beentered in the above relationship for determining the surface area F_(a)of the waste piece, instead of the total weight of the stack to beplaced into the mold. The size of the not-included further blank wouldhave to be cut out from the waste piece, it being necessary for thisblank to be cut to size with respect to its surface content in inverseproportion to the determined basis weight of the reference blank.

[0054] For the sake of completeness, a modification of the cuttingmethod according to FIG. 6 should also be mentioned, a useful blank 33of the same shape, trapezoidal in rough approximation, being assumedhere. This modification of the process manages with a much smalleramount of trimmed-off waste. To be precise, on the one hand the usefulblanks that are trapezoidal in rough approximation would have to beplaced into the prepreg web in such a way that the two straight andmutually parallel side edges of the useful blanks come to lie parallelto the side edges of the prepreg web; the feed of the prepreg web wouldconsequently have to be imagined from the left or right side in FIG. 6.Moreover, the width of the prepreg web would have to be chosen such thatit coincides with the corresponding dimension of the largest requireduseful blank—which is represented in FIG. 6 by a dash-dotted line; itwould also be conceivable to arrange two sequences of reference partsparallel next to one another on the prepreg web. On the other hand, theuseful blanks following one another in the longitudinal direction of theweb are cut out from the prepreg web alternately from one side then theother, so that the large dimension, parallel to the longitudinaldirection of the web, of one useful blank always coincides with thesmall dimension of the next-following useful blank.

[0055] In the case of such a variant of the method, the reference blankthat is to be used is a blank already cut to size in a way correspondingto the desired free form, which is dimensioned to be large enough to bejust right with respect to its surface-area content in the case of thelowest basis weight of the prepreg web, i.e. the basis weight lying atthe lower end of the range of variation, and has the required desiredweight. By temporarily lifting the end of the prepreg web off thecutting table, which is at the same time the weighing plate, it ispossible to determine the actual weight of this reference part, which isgenerally too high in comparison with the desired weight to bemaintained. In a way similar to that already described in connectionwith FIG. 6, the excess weight of the reference part in question isconverted into a new blank contour, three of which are indicated in FIG.6 by differently drawn line traces. The reference part is specificallyreduced in weight by a more or less wide edge trim similar to theoutline shape of the reference part and, as a result, the desired weightof the useful blank 33 is exactly brought about. Apart from theform-dependent trim, only a relatively narrow, weight-dependent edgestrip is then obtained as trimmed-off waste.

[0056] The foregoing disclosure has been set forth merely to illustratethe invention and is not intended to be limiting. Since modifications ofthe disclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

1-11. (Canceled)
 12. A method for manufacturing a plurality of SMCcomponents comprising: cutting a plurality of blanks from a singleprepreg web, wherein the blanks are directly adjacent to one another inthe prepreg web prior to cutting; stacking said plurality of blanks toform a multilayer prepreg stack; placing said multilayer prepreg stackinto a heated mold of a molding press; flow molding the prepreg stack byat least partially closing the mold to form a SMC component; andthermally curing said SMC component in said mold, wherein said mold isclosed during curing, wherein at least one blank of said plurality ofblanks is a reference blank, said reference blank being cut to size tohave a reference shape and reference surface area (Fr), and wherein atleast one additional SMC component is formed from at least oneadditional multilayer prepreg stack, each said at least one additionalmultilayer prepreg stack including an additional reference blank havingsaid reference shape and said reference surface area, each additionalreference blank being separately weighed after cutting; wherein theweight of said reference blank G_(r,actual), the surface area of saidreference blank F_(r), and the desired total weight G_(g) for all blanksin a multilayer prepreg stack are used to determine a total surface areaF_(u) for the non-reference blanks in said multilayer prepreg stack inaccordance with the relationship F_(u)=F_(r)·(G_(g)/G_(r,actual)−1), orin accordance with a relationship derived therefrom and identical inprinciple; and wherein the other blanks of said plurality of blanks arenon-reference blanks, said non-reference blanks being cut from a portionof the prepreg web directly adjacent to the reference blank, and saidnon-reference blanks having a total surface area F_(u).
 13. The methodas claimed in claim 12, wherein the blanks for said multilayer prepregstack and said one or more additional multilayer prepreg stacks are cutfrom said single prepreg web.
 14. A method for manufacturing a pluralityof SMC components, comprising: cutting a plurality of single-layerblanks from a single prepreg web; placing each said single-layer blankinto a heated mold of a molding press; flow molding each blank by atleast partially closing the mold to form a SMC component; and thermallycuring each said SMC component in said mold, wherein said mold is closedduring curing; wherein cutting a plurality of single-layer blankscomprises: cutting a plurality of reference blanks having a referenceshape and reference surface area from the prepreg web; determining anexcess surface area F_(a) for each reference blank based on the actualweight of the reference blank G_(r,actual), the surface area of thereference blank F_(r), and desired weight of the single-layer blankG_(n), in accordance with the relationshipF_(a)=F_(r)·(1−G_(n)/G_(r,actual)), or in accordance with a relationshipwhich is derived therefrom and identical in principle; and cutting awaste piece having said surface area F_(a) from an edge of eachreference blank to form single-layer blanks having the desired weightG_(n) and a shape corresponding to a desired single-layer blank shape.15. The method as claimed in claim 12, wherein the non-reference blanksin said multilayer prepreg stack are rectangular and congruent inrelation to one another, wherein a dimension (l) of the side of therectangle is the same for the non-reference blanks of said multilayerprepreg stack and for the non-reference blanks of said at least oneadditional multilayer prepreg stack, and wherein a dimension (b)transverse to the dimension (l) is dimensioned individually for thenon-reference blanks in each multilayer prepreg stack in accordance withthe relationship b=F_(r)·(G_(g)/G_(r,actual)−1)/l·n, where n denotes thenumber of non-reference blanks.
 16. The method as claimed in claim 15,wherein the dimension (l) is aligned parallel to the longitudinaldirection of the prepreg web and the dimension (b) is alignedtransversely to the longitudinal direction of the prepreg web.
 17. Themethod as claimed in claim 12, wherein the surface area of the referenceblank is approximately 20 to 60% of a total surface area of all blanksof the multilayer prepreg stack.
 18. The method as claimed in claim 12,wherein the blank lying at the bottom of the multilayer prepreg stack isthe reference blank.
 19. The method as claimed in claim 12, wherein thereference blank and the non-reference blanks of the multilayer prepregstack are cut out next to one another from the prepreg web, transverselyto the longitudinal direction of the prepreg web, in such a way that thespace required in the longitudinal direction of the prepreg web is assmall as possible.
 20. The method as claimed in claim 12, wherein saidcutting is performed by a robot-guided cutting tool, and wherein theprepreg web rests on a hard base.
 21. The method as claimed in claim 12,wherein said cutting is performed by a circular saw blade, said circularsaw blade performing a rotary oscillating movement with more than 15,000cycles per minute, and also having rotary oscillating displacements,wherein said displacements are greater than a tooth pitch of saidcircular saw blade.
 22. The method as claimed in claim 21, wherein saidcircular saw blade performs a rotary oscillating movement having fromabout 20,000 to about 30,000 cycles per minute.
 23. The method asclaimed in claim 21, wherein said displacements are smaller than thethickness of the prepreg web.
 24. The method as claimed in claim 21,wherein during the cutting by the rotary oscillating circular saw blade,the prepreg web is supported by a smooth, continuous base that is freefrom joints, wherein said base is composed of a harder material than thecutting teeth of the circular saw blade.
 25. The method as claimed inclaim 12, wherein the prepreg web is stripped of coverings or protectivefilms prior to cutting.